For high pH aqueous systems in equilibrium with the atmosphere, the speciation of U(VI) is dominated by negatively charged uranyl-carbonate and uranyl-hydroxide complexes, which are poorly adsorbed by most natural and engineered surfaces. As a result, the remediation of U-contaminated environments is particularly challenging above pH 8. In this study, we demonstrate the extraordinary capacity of many-layered graphene oxide (MLGO) nanosheets to adsorb U(VI) through pH 11. MLGO nanosheets owe their especially reactive surfaces to an extremely high surface area to mass ratio and the presence of a variety of proton-active functional groups. With ionic strengths (IS) held constant (between 5 and 100 mM), we measured the adsorption of 3 ppm U onto MLGO as a function of both MLGO concentration and pH. Adsorption increased with decreasing IS between pH 7.8 and pH 9, and significant adsorption (>90 percent) was observed in systems containing as little as 40 mg MLGO L-1. Similarly, >90 percent adsorption occurred in systems between pH 3.5 and 9.7 at each IS. Precipitation and/or container losses of U in corresponding MLGO-free control systems surpassed 10 percent above pH 9.7, thereby complicating the further analysis of U-MLGO complexation. However, U removal from MLGO-bearing systems as high as pH 10.5 exceeded 80 percent, indicating that most of the U removed in these systems could be attributed to U(VI) adsorption onto MLGO surfaces. The high extent of U(VI) adsorption under high pH conditions strongly suggests adsorption of uranyl-hydroxide and uranyl-carbonate complexes onto the MLGO surface. Using pKa values and binding site concentrations determined during IS-specific potentiometric titrations, we use these data to construct surface complexation models that constrain the stoichiometry and thermodynamic stability constants associated with each complexation reaction. The results of this study indicate that MLGO is an extremely effective sorbent of U(VI) species in high pH environments.

Significant high pH adsorption of aqueous uranyl complexes onto many-layered graphene oxide

246th National Meeting (2013)

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